So it happened, long time Joss Whedon and James Gunn collaborator, Nathan Fillion teased his involvement in Marvel's The Guardians of the Galaxy. He let it slip at Wizard World Comic-Con in St Louis. Start at 38:07 for the good bits:

To celebrate, I'm counting down the Top 5 Roles we want Fillion to play that he never will, ordered from most to least likely.